Field of the Invention
The present disclosure relates to a reading apparatus that reads a sheet. Particularly, the present disclosure relates to a technique that allows a sheet to be read with high accuracy.
Description of the Related Art
Reading apparatuses such as scanners have been widely used. A reading apparatus includes a line sensor, and reads a sheet using the line sensor. There are two line sensor-based reading methods: one method uses three line sensors to read a sheet (hereinafter, referred to as a “first reading method”); and the other method uses one line sensor to read a sheet (hereinafter, referred to as a “second reading method”).
The following describes the first reading method. A color filter that transmits red light is provided in a first line sensor among the three line sensors. A color filter that transmits green light is provided in a second line sensor. A color filter that transmits blue light is provided in a third line sensor. A reading apparatus includes a white light source, and the white light source irradiates a sheet with white light. Light reflected off the sheet enters the first to third line sensors. The first line sensor receives the light that has been transmitted through the color filter, and then generates an image representing red components of the sheet. The second line sensor receives the light that has been transmitted through the color filter, and then generates an image representing green components of the sheet. The third line sensor receives the light that has been transmitted through the color filter, and then generates an image representing blue components of the sheet. The reading apparatus then generates a color image by combining the images of the respective colors.
JP 2008-028617 A (Patent Document 1), JP 2013-229704 A (Patent Document 2), and JP 2011-109625 A (Patent Document 3) disclose techniques that apply the first reading method.
The following describes the second reading method. In a reading apparatus adopting the second reading method, a light emitting diode (LED) that emits red light, an LED that emits green light, and an LED that emits blue light are provided. The reading apparatus consecutively switches over the light emission timing of each LED, causing light of each LED to be emitted to a sheet to be read. A line sensor receives the red light reflected off the sheet, and generates an image representing red components of the sheet. The line sensor receives the green light reflected off the sheet, and generates an image representing green components of the sheet. The line sensor receives the blue light reflected off the sheet, and generates an image representing blue components of the sheet. The reading apparatus then generates a color image by combining the images of the respective colors.
Generally, a charge-coupled device (CCD) is adopted as a line sensor in the first reading method. In the second reading method, a contact image sensor (CIS) is adopted as a line sensor. Since the CCD is more expensive than the CIS, the second reading method can be implemented with lower costs than the first reading method.
On the other hand, the reading accuracy of the first reading method is higher than that of the second reading method. One of the reasons is a difference in illumination light. Light of a white LED that the first reading method adopts has a spectral distribution ranging over a visible region from purple to red. By contrast, in the second reading method, an LED of each color emits red light, green light, and blue light. Therefore, these LEDs have a few wavelength components other than their respective red, green, and blue wavelength components. As a result, the color reproducibility of the second reading method is lower than that of the first reading method.
Accordingly, it is desired that a reading apparatus have both advantages: the cost advantage of the first reading method and the advantage of the reading accuracy of the second reading method.